This is a two-arm, parallel, randomized, double-blind, placebo-controlled Phase 4 multicenter trial to compare the whole day efficacy of atomoxetine versus placebo in children aged 6 through 12 years with Attention-Deficit/Hyperactivity Disorder (ADHD) treated in an inpatient, day-patient and outpatient setting in Germany. Core symptoms will be measured during once or bi-weekly visits, three times per visit-day, by a computer based Continuous Performance Test. Following an initial 3-28-day screening and washout phase, patients will be assigned to double-blind treatment with atomoxetine or placebo. In the verum arm, a one-week atomoxetine treatment period with the 0.5 mg/kg per day lead-in dose will be succeeded by a 7 week period at the target dose of 1.2 mg/kg per day.